Plum Pox Virus and Sharka: a model Potyvirus and a Major Disease

PPV AS A TOOL IN BIOTECHNOLOGY 
Plant viruses are object of interest not only because of the harm they cause to crops. 
Viral infections can enhance the esthetical value of ornamental plants (Saunders et al., 
2003, Garber, 1989) and viruses may even establish interactions mutually beneficial 
to the virus and the host (Roossinck, 2005). Although there are no reports indicative 
of beneficial effects of PPV, genetic engineering has allowed to modify and use PPV, 
or parts of it, as valuable biotechnological tools.
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The availability of functional full-length cDNAs of the PPV genome 
(Riechmann et al., 1990, Maiss et al., 1992, Szathmary et al., 2009, 
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Hyperlink reference not valid., López-Moya & García, 2000) has facilitated the 
development of PPV-based vectors to express either small peptides fused to the viral 
CP or independent proteins (García et al., 2006). Several vectors developed to express 
epitopes of foreign agents at the surface of PPV virions differed in their tolerance to 
inserted sequences and in antigenicity and immunogenicity of the expressed epitopes 
(Fernández-Fernández et al., 2002b, Fernández-Fernández et al., 1998).
PPV-based vectors allowing to express whole independent proteins have also 
been constructed, using as insertion sites the P1/HCPro or the NIb/CP junctions 
(García et al., 2006). These vectors have been used to express reporters that facilitate 
the monitoring the viral infection (Guo et al., 1998b, Dietrich & Maiss, 2003, Ion-
Nagy et al., 2006, Lansac et al., 2005), but also antigenic proteins to produce 
recombinant vaccines (Fernández-Fernández et al., 2001). 
Viral vectors can be expressed in transgenic plants transformed with full-length 
cDNA copies of the viral genome. These amplicons combine the genetic stability of 
transgenic plants with the elevated replication rate of viruses. PPV amplicons have 
been developed in N. benthamiana, but showed important constraints that limit their 
utility (Calvo et al., 2010). A PPV amplicon has been used to design a method to 
control virus expression by regulating the temperature during plant transformation and 
its subsequent culture, which could help to reduce such limitations (Dujovny et al., 
2009). 
The protease domain of the protein NIa of PPV has demonstrated a notable 
biotechnological interest since its high efficiency and specificity makes it very 
attractive for processing of fusion proteins both in vitro (Zheng et al., 2008, Pérez-
Martín et al., 1997) and in vivo (Zheng et al., 2012).